Evaluating the suitability of Li8PbO6 as a tritium breeding material using first-principles simulations

Davies, Andrew and Murphy, Samuel (2024) Evaluating the suitability of Li8PbO6 as a tritium breeding material using first-principles simulations. PhD thesis, Lancaster University.

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Abstract

A desire to move away from ceramic breeder materials that display a dependence on the use of a beryllium multiplier, has seen octalithium compounds and particularly, Li8PbO6, being considered for application in a future fusion reactor. Although, there is little understanding of many of the basic properties of the material, importantly how it might behave as a tritium breeding material. By virtue of proximity to the fusion plasma, the extreme operating environment the breeding material will be subject to inherently gives rise to a significant population of defects throughout the operational lifetime. This has implications for the not only the macroscopic physical properties of the material, but also the mechanisms for tritium accommodation and release. Modern first-principles simulations such as Density Functional Theory are widely used to study the fundamental properties of crystalline materials, including the behaviour of both intrinsic and extrinsic point defects. In this thesis, DFT is used to assess the feasibility of Li8PbO6 as a breeder material and aims to provide a comprehensive understanding of the underlying defect chemistry and the mechanisms for tritium release. But also, when combined with simple thermodynamics, how the defect population controls the underlying stoichiometry, and how ultimately, the burn-up of lithium may result in the stability of octalithium phase being lost. It is shown that the defect chemistry, given the intrinsically high concentration of lithium in Li8PbO6, is largely dominated by lithium vacancy defects (namely VLi−1), regardless of the operating conditions considered. It is expected Li8PbO6 when accounting for point defect populations will only be stable under Li-rich stoichiometries and may begin to undergo a phase transformation into Li4PbO4 as lithium is depleted throughout the operational lifetime. Migration barriers for tritium diffusion as an interstitial and bound to a lithium vacancy were found to be comparatively low compared to other lithium ceramics. The low migration barriers for the defect complex in particular suggest the lithium vacancy performs a collaborative role in assisting tritium escape from the bulk crystal, which suggests aging of the blanket will be of low significance in comparison with other leading candidate ceramics. The works presented in this thesis provide a baseline theoretical understanding of the potential performance of Li8PbO6 to operate as a breeding material. It is hoped the works presented can be experimentally validated, and more importantly to serve as encouragement for others to explore the viability of other, previously dismissed materials.